When her normally boisterous dog Max began moping around the house, Maureen Burns wondered if he was coming down with something.
But it turned out she was the one who was sick - and she believes the nine-year-old collie cross was deliberately acting out of character to alert her.
After Max began sniffing Mrs Burns's breath and then gently nudging her right breast, the 64-year-old examined herself and discovered a small lump in the same breast.
My hero: Maureen Burns with Max the dog who she thinks 'sniffed-out' her cancer
She was referred to hospital by her GP and a biopsy later confirmed the former draughtsman had a cancerous tumour.
The case supports growing evidence that some breeds of dogs appear to have the ability to 'sniff out' diseases like cancer.
Mrs Burns said: 'When the nurse told me I had breast cancer my first response was, 'I know, my dog told me!'. I expected her to laugh but instead she told me she had heard of similar cases.
'Max is usually such an excitable, loving animal but he became very sad and had stopped doing all the things he used to - such as sharing our bed or jumping on my lap for cuddles. Instead he would touch my breast and back off unhappily.'
Best pals: Maureen walking Max - she says the dog 'helped save her life'
Mrs Burns, who lives in Rugby, Warwickshire, with husband Roger, 66, a retired engineer, has since had the lump removed and her prognosis is excellent.
She said she finally realised there was something wrong last May when Max watched as she examined herself in the mirror.
She said: 'I felt a lump but I wasn't unduly worried as I'd had a lump 20 years ago and it had proved to be benign.
'I'd also had a routine mammogram which came back negative just 15 months earlier.
'As I felt it I just happened to look over at Max, who was lying on my bed. Our eyes met and I just remember he looked so sad.
'I knew in that instant that something was badly wrong.'
Mrs Burns had an operation to remove the inch-wide lump a few weeks later.
Doctors also removed four lymph nodes from her underarm, in case the cancer had spread, but they came back all-clear.
The Burns, who have no children, said that while their other pet, a retired greyhound called Grace, had behaved no differently throughout the health scare, when Mrs Burns returned home after having the operation in June, she was greeted by Max 'acting like he was a puppy again'.
She said: 'It was as though he knew I was okay again. He stopped sniffing me and became very playful again. I owe him so much.
'Max helped to save my life - he was right all along.'
A nose for disease
Experts believe dogs can be trained to detect cancer.
A dog's sense of smell is 100,000 times more powerful than that of a human.
In 2004, a study by Buckinghamshire Hospitals Trust and the charity Cancer and Bio-detection Dogs, found the pets could detect bladder cancer in urine samples.
The research programme is to be expanded to other forms of cancer.
Cancer cells are known to produce chemicals called Volatile Organic Compounds which give off distinct odours which dogs are believed to react to.
The dogs are trained to point their nose at sample pots they believe are cancerous.
Saved by the dog-tor: Pet collie 'sniffed out' that owner had breast cancer | Mail Online